Re: Bipedalism and theorizing... was Re: Morgan and creationists

John Waters (jdwaters@dircon.co.uk)
4 Oct 1996 12:16:36 GMT

Gerrit Hanenburg <ghanenbu@inter.nl.net> wrote in article
<Dyo3Cu.GH1@inter.NL.net>...

> That's why I think that reorganization of the pelvic region
and lower
> limbs is crucial,and happened early in hominid evolution.
> It may have started during a stage when the main function of
> bipedalism was postural and not yet locomotory.

JW: I tend to agree. It is clear that the early changes were
either purely postural, or a combination of postural changes and
locomotory changes, or purely locomotory changes.

As you know, there is an existing hypothesis on bipedalism which
considers that it started through a change in posture, when the
specie fed on fruit or leaves which were four to five feet off
the ground. I find this quite untenable, for the simple
reason that the habitat€s herbivorous browsers and graziers
would eat all the food at this level. The main advantage of
being a primate is that such animals can reach arboreal foods
which are beyond the reach of browsers and graziers.

However, in the context of an adaptation to a more marginal
habitat, it is clear that there would be advantages accruing to
individuals who were taller and lighter. Such individuals could
reach higher at the tops of trees, and move safely further out
on
the branches to reach less accessible fruit and nuts. In
addition, these taller individuals would tend to stand
straighter on the ground, so the energy losses involved in
ground level locomotion would be lower.

The problem with this scenario is that a taller and lighter body
structure implies a slimmer pelvis. This would lead to a change
in the head to body ratio and a consequential extension in the
period of helplessness of the baby after birth. Indeed, any
change of posture would seem to lead to this conclusion. What do
you think?

I can understand your problems with the clades, although I think
the whole business of cladism has been taken to extremes. If
only we had another Mendeleef. (Table of Elements).

However, which ever scenario is adopted as regards the
locomotory behaviour of the LCA, a competent theory of human
evolution must be able to answer the following questions.

If the LCA was a knuckle walker and the Hominids inherited this
form of ground level locomotion, why did the Hominids switch to
bipedalism?

Alternatively, if the LCA was bipedal and the Hominids
inherited this form of locomotion, why did the Hominids not
switch to knuckle walking when they needed to traverse greater
distances on the ground?

If knuckle walking was the appropriate evolutionary change for
the closely related Gorilla/Chimp species, why did Hominids not
evolve the same technique?

According the Helpless Baby Theory, the answer to both questions
is the same. The Hominids were a marginal variant specie whose
postural adaptations led to an extension in the period of
helplessness of the baby. Maternal instinct forced the
nursing female to carry her new-born baby in her arms, and this
led either to the adoption of bipedalism, or prevented the
specie from switching to knuckle walking.

However, this still does not resolve the question of how the
nursing female would carry a three year old infant. In this
regard, it is fair to assume that a slimmer, lighter adult would
have a slimmer, lighter infant. What are the cost/benefit ratios
here?

In addition, there is the problem of infant thermoregulation.
This would manifest itself in the context of increasing
distances travelled on the ground. It might appear that this
would only affect the infant while it was helpless, and
therefore held in its
mothers arms. But conceivably this could also affect infants who
gripped the fur/skin of the nursing female, in so far as the
vertical posture of the mother would bring the infant into
closer proximity with her body. Have you thought about this?

John.